Some people are lucky enough to have near-perfect vision which requires no optical correction. However, a great many people suffer from one or more visual impairments. For example, some people suffer from hypermetropia (farsightedness) in which light images nearer than a certain distance cannot be focused properly on the retina, but instead are focused behind it. Other people suffer from myopia or nearsightedness. The visual impairments just described typically can be corrected using contact lenses having spherical surfaces. However, a good many other visual impairments require contact lenses which have aspherical surfaces. For example, astigmatism, a common malady, requires the use of non-axisymmetric lens surfaces. Astigmatism is a refractive defect in which the curvature of the eye is different in one direction than it is in another direction. To correct this then, a contact lens is required having surfaces with differing curvature radii in order to compensate for the astigmatism. Oftentimes, the astigmatism is coupled with nearsightedness or farsightedness so that the contact lenses required need to correct both conditions.
In the past, one way of providing contact lenses with both spherical surfaces and aspherical surfaces has been to machine and polish a spherical correction onto a concave inner surface of a lens and thereafter to crimp the lens in a holding device to oblate the lens. The thus-oblated lens is then machined and polished on the convex outer surface. After the lens is released from the crimping device, the lens returns to its uncrimped configuration, leaving a toric surface on the outer surface of the lens. This process is labor-intensive and requires great precision in the crimping.
It has been recognized in the art that the crimping process is less than desirable and efforts have been made to provide for the manufacture of toric lenses without crimping. For example, U.S. Pat. No. 4,680,998 of Council, Jr. (assigned to Bausch & Lomb, Inc.) relates to "TORIC LENSES, METHOD AND APPARATUS FOR MAKING SAME". The '998 patent describes a lathe apparatus in which a lens blank is chucked to a spindle and the spindle rotated. A cutting tool is supported on a rotary quadrant whose rotational swing moves the cutting tool arcuately across the face of the lens blank. In this way, the cutting tool is moved through a predetermined tool path in order to cut the lens as the spindle rotates. Furthermore, the precise, momentary angular position of the lens is monitored as the lens rotates on the spindle and the cutting tool is oscillated as necessary in synchronization with the angular position of the lens in order to provide a non-spherical surface on the lenses. Of course, normally, as a workpiece is turned in a conventional lathe, the resulting product has a surface of revolution matching the path taken by the tip of the cutting tool. By selectively oscillating the cutting tool during the rotation of the workpiece, some parts of the surface can be truly a surface of revolution (axisymmetric), while other parts can be non-axisymmetric. The '998 patent discloses the use of an electric motor) to effect a rotary oscillation of a vertically oriented tool post (100) about the motor shaft (99). With the cutting tool (36) being mounted on the tool post, rotary oscillation of the cutting tool is effected. One disadvantage of this arrangement is that the useful magnitude (useful stroke) of the cutting tool's oscillations is severely limited because as the magnitude of the oscillations increases, the position of the cutting tool varies relative to the lens blank. This tends to limit the lenses that can be made with this arrangement.
U.S. Pat. Nos. 4,884,482 and 4,947,715, each of Council, Jr. (and each assigned to City Crown, Inc. of Tampa, Fla.), relate to method and apparatus for cutting an aspheric surface on a workpiece. The apparatus and method described in the '482 and '715 patents is quite similar to that described in the '998 patent. However, in the '482 and '715 patents, instead of oscillating the cutting tool as is described in the '998 patent, the workpiece is oscillated (by oscillating the spindle) in an effort to improve the effective useful stroke of the oscillations between the cutting tool and the workpiece. However, oscillating the workpiece is somewhat problematic. In recent years, another lathe apparatus has been placed on the market by Rank Pneumo (a division of Rank Taylor Hobson of Keene, N.H.) for generating toric lenses. The Rank Pneumo lathe apparatus includes an X-Z 2-axis (translating) quadrant. The Rank Pneumo lathe apparatus utilizes a tool servo mechanism for the production of non-axisymmetric shapes, including torics, off-axis conics, and other free form surfaces. This lathe apparatus provides a rapidly positionable linear motion (Z') which is parallel to the traditional Z axis of travel for the cutting tool. The motion is accurately coordinated with the angular position of the work-holding spindle and the linear motions of the X and Z axes to produce preprogrammed non-axisymmetrical shapes. One advantage of this apparatus over what is described in the '998 patent is that the oscillation is translational and is effected by using dual piezoelectric actuators with a lever mechanism for amplifying the motion of the actuators. The piezoelectric elements have a naturally high frequency which allows toric lens geometries to be machined at spindle speeds of up to approximately 6,000 rpm. As a result, relatively high production rates can be achieved.
The apparatus made by Rank Pneumo has an actual maximum stroke of between 0.2 mm and 0.4 nun, depending upon the rate of oscillation. Near the center of the lenses to be cut, such a limited motion often is not a serious drawback. However, out nearer the edges of the lenses, the effective stroke of the known Rank Pneumo arrangement becomes smaller due to the fact that the stroke is oriented at a substantial angle relative to the thickness of the lenses at that point.
Accordingly, it can be seen that a need yet remains for a lathe apparatus and method which allows the production of non-axisymmetric surfaces on lenses and which provides a greater effective stroke over known apparatus and techniques. It is to the provision of such a lathe apparatus and method that the present invention is primarily directed.